Astronomy
&
Astrophysics
A&A 498, 139–145 (2009)
DOI: 10.1051/0004-6361/200810742
c ESO 2009
On the metal abundances inside mixed-morphology supernova
remnants: the case of IC 443 and G166.0+4.3
F. Bocchino1 , M. Miceli2 , and E. Troja3,4
1
2
3
4
INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: [email protected]
Consorzio COMETA, via S. Sofia 64, 95123 Catania, Italy
Dipartimento di Scienze Fisiche ed Astronomiche, Sezione di Astronomia, Università di Palermo, Piazza del Parlamento 1,
90134 Palermo, Italy
INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Palermo, via Ugo la Malfa 153, 90146 Palermo, Italy
Received 4 August 2008 / Accepted 10 January 2009
ABSTRACT
Context. Recent developments in the study of mixed morphology supernova remnants (MMSNRs) have revealed the presence of
metal-rich X-ray emitting plasma inside a fraction of these remnants, a feature not properly addressed by traditional models for these
objects.
Aims. Radial profiles of thermodynamical and chemical parameters are needed for a fruitful comparison of data and model of
MMSNRs, but these are available only in a few cases.
Methods. We analyzed XMM-Newton data of two MMSNRs, namely IC 443 and G166.0+4.3, previously known to have solar metal
abundances, and performed a spatially resolved spectral analysis of the X-ray emission.
Results. We detected enhanced abundances of Ne, Mg and Si in the hard X-ray bright peak in the north of IC 443, and of sulphur
in the outer regions of G166.0+4.3. The metal abundances are not distributed uniformly in both remnants. The evaporating clouds
model and the radiative SNR model fail to reproduce consistently all the observational results.
Conclusions. We suggest that further deep X-ray observations of MMSNRs may reveal more metal-rich objects. More detailed models
including ISM-ejecta mixing are needed to explain the nature of this growing subclass of MMSNRs.
Key words. ISM: supernova remnants – ISM: dust, extinction – X-rays: ISM – X-rays: individuals: IC 443 –
X-rays: individuals: G166.0+4.3 – ISM
1. Introduction
Mixed-morphology supernova remnants (MMSNRs) have been
traditionally defined as remnants with a shell morphology in radio and a centrally peaked morphology in the X-ray band characterized by a thermal spectrum (Rho & Petre 1998). The origin
of this atypical morphology is controversial. Traditional models
rely mostly on the effects of thermal conduction (e.g., Cox et al.
1999; Shelton et al. 1999; White & Long 1991), but there are
also models that invoke projection effects (Petruk 2001). Tilley
et al. (2006) attempted to improve previous analytical models by
using a set of hydrodynamical simulations designed to reproduce
the morphology observed in MMSNRs.
In parallel to these theoretical efforts, there is growing observational interest about MMSNRs, which has led in a few cases
to useful comparisons with models. Slane et al. (2002) favor the
evaporating clouds White & Long (1991) model for G290.1-0.8
(rather then the radiative model of Shelton et al. 1999), but problems remain in the comparison of the surface brightness profiles.
Lazendic & Slane (2006) also found a reasonable agreement between both models and X-ray observations of CTB1 and HB21,
but they pointed out that the predicted central density was below
the derived densities of the emiting plasma. In contrast, Rho &
Borkowski (2002) argued that the White & Long (1991) evaporating cloud model cannot explain the strong radiative shock
front in the MMSNR W28, and that the radiative model could
not reproduce the strong thermal variation found in this remnant
(see also Chevalier 1999, for a critical analysis of the evaporating cloud model of MMSNRs).
Several authors proposed that some of the X-ray emission
of MMSNRs may be due to thermal plasma of high metal
abundances, as in the case of stellar ejecta inside young historical SNRs (e.g., Shelton et al. 2004; Chen et al. 2008), or
in other Galactic (e.g., Vela SNR, Miceli et al. 2008; Cygnus
Loop, Katsuda et al. 2008; Puppis A, Hwang et al. 2008) and
Magellanic Cloud SNRs (e.g., 0103-72.6, Park et al. 2003a;
N49B, Park et al. 2003b). This important result was addressed
and summarized by Lazendic & Slane (2006), who compiled a
sample of 26 MMSNRs, 10 of which appear to show enhanced
metal abundances in their X-ray spectrum. We emphasize that
the presence of enhanced metal abundances inside MMSNRs has
not been addressed in detail in models of this sub-class of remnants. Published models with thermal conduction do not yet take
account of the mixing of ejecta with the circumstellar and interstellar medium, so the emerging class of metal-rich MMSNRs is
still not properly understood. This is why comparisons between
models and observation have focused mostly on morphological
issues, with the general comment that the presence of the additional ejecta component may reconcile the discrepancy between
model and observed profile (as in the case of W44, Shelton et al.
2004).
The mixed morphology category of SNRs (and especially
the emerging subclass of metal-rich MMSNRs) can be understood more clearly if we have a large sample of objects with well
Article published by EDP Sciences
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F. Bocchino et al.: Metal abundances in MM SNRs
studied characteristics. In particular, the accurate determination
of density, temperature, and abundances profile for MMSNRs
would allow us to perform an accurate comparison both with traditional models and with a new magnetohydrodynamical (MHD)
model of shocked ISM and ejecta inside MMSNRs, which is the
subject of a forthcoming publication.
We systematically reviewed the MMSNRs listed in Lazendic
& Slane (2006) with the primary goal of measuring the abundances in the inner region of the remnants and other X-ray properties. In this paper, we review the XMM-Newton archive observations of two bright MMSNRs, IC 443 and G166.0+4.3, and
attempt to derive the distribution of temperature and metal abundances inside these remnants. Both our targets are mentioned
in the list of Lazendic & Slane (2006) as MMSNRs with standard metal abundances. However, Troja et al. (2006) and Troja
et al. (2008) studied in detail the X-ray emission of IC 443,
and found enhanced metal abundances in a remarkable ring feature around the pulsar wind nebula, in the southern part of the
remnant, which they attributed to emission from shocked ejecta.
The compilation of MMSNRs with enhanced metal abundances
needs therefore to be updated with new results from deep X-ray
observations of MMSNRs. In the light of these new observational results, traditional models should be verified and revised
as necessary.
The paper is organized as follows. In Sects. 2 and 3, we
present the results obtained form XMM-Newton archive observations of IC 443 and G166.0+4.3, respectively. In Sect. 4, we
present the comparison of our results with the standard models for MMSNRs, namely the evaporating clouds White & Long
(1991) model and the radiative SNR model of Cox et al. (1999).
In Sect. 5, we present the conclusion of our work.
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2. IC 443
IC 443 has been extensively studied and observed in several
bands and, for this reason, is a case study for the interaction between SNR shocks and molecular clouds. Recent X-ray studies include those of Troja et al. (2008), Bykov et al. (2008),
and Troja et al. (2006), while infrared and radio studies include Noriega-Crespo et al. (2008), Bykov et al. (2008), Lee
et al. (2008), Rosado et al. (2007), Leahy (2004), and references therein. The remnant was classified as mixed-morphology
by Rho & Petre (1998), based on observations with the ROSAT
satellite. Troja et al. (2006) and Troja et al. (2008) confirmed
the centrally peaked morphology between 0.5 and 5 keV, and a
bright X-ray peak close to 06h 17m 09s +22◦ 45m in the 1.4–5 keV
(Fig. 1). However, they note that, at very soft energies (0.3–
0.5 keV), the remnant exhibites an incomplete shell extending
from N to E, corresponding to one of the regions of interaction
with a large cloud. They also investigated the spatial distribution of the metals inside this remnant, using the technique of the
equivalent width images and demonstrated that the distribution
of Si and S is far from being homogeneous. In particular, a ring
of metal-rich plasma surrounding the pulsar wind nebula is evident in the equivalent width images of these two elements (see
their Fig. 3). The northern region of the remnant, coincident with
a maximum in the hard X-ray image (Fig. 1), is also characterized by a large value of the Si and S equivalent width, suggesting
that it has a higher metal abundances with respect to surrounding
regions. In the following, we focus on this region.
The data that we use in this work are part Calibration and
Performance Verification phase of the XMM-Newton satellite
(Jansen et al. 2001), for a total of 6 observations of IC 443. We
have also used the public archive observation 0301960101. The
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Fig. 1. Top: XMM-Newton EPC mosaic of the northern region of the
IC 443 supernova remnant in the 1.4–5.0 keV band. The image is background subtracted and vignetting corrected. The pixel size is 10 arcsec.
Five contours levels at 8, 10, 12, 14, and 16×10−4 cnt s−1 pix−1 are overlaid. These contours have been used as boundaries for the 5 spectral extraction regions discussed in the text. Bottom: same region at 1420 MHz
(total intensity map) with X-ray contours overlaid (adapted from Leahy
2004).
datasets that we used are the same datasets used by Troja et al.
(2008). We screened both PN and MOS data using the algorithm
suggested by Snowden & Kuntz (2007).
To study the profile of the thermodynamical and chemical
parameters of this remnant, we chose 5 spectral regions defined
in terms of contours of iso-surface brightness, centered on the
bright northern region, as shown in Fig. 1. For each region, we
computed the mean distance to the X-ray peak by finding the
average distance for all pixels in that region. XMM-Newton PN
and MOS spectra were extracted using the software SAS 7.1 and
corrected for detector non-uniformity using the task evigweight.
Troja et al. (2006) demonstrated that the X-ray emission in the
IC 443 interior consists of two thermal components, a soft one
of temperature in the range 0.3–0.7 keV, in a non-equilibrium of
F. Bocchino et al.: Metal abundances in MM SNRs
141
IC443 Third contour region
PN (black), MOS1 (red) and MOS2 (green)
Si
S
Ar
Ne
O
Residuals
−2 0
2
Counts s−1 keV−1
0.01
0.1
1
10
Mg
1
2
Energy (keV)
Fig. 2. XMM-Newton EPIC spectrum of the third contour region in
Fig. 1 (average distance 2.8 in Fig. 3). The best fit two thermal component model is shown as continuous line (with residual in lower panel),
while individual components are shown in dashed black lines for the PN
spectrum only. Emission lines from O (0.6–0.8 keV), Ne (∼1 keV), Mg
(∼1.4 keV), Si (1.8–2 keV), S (∼2.5 keV) and Ar (∼3 keV) are visible.
Ionization (NEI) state, and a second with kT = 1.1−1.8 keV,
in equilibrium of ionization (CIE) state. The soft component
has been associated with the shocked interstellar material, and
the hot component with the shocked stellar ejecta. Therefore,
we used the same emission models, namely the mekal model
(Mewe et al. 1985) and the VNEI (Borkowski et al. 2001)
model of XSPEC v11.3.2 (Arnaud 1996). We fixed the chemical
abundances of the soft component to solar values, and allowed
the abundances of the hot component to vary. The adopted models represent an approximation to the true conditions inside this
complex remnant. However, this approach allows us to measure
quantitatively the emission-measure-weighted temperature and
metal abundances of each component in a spectrum. By using
the region layout shown in Fig. 1, we may also derive meaningful constraints on the measured quantities related to the radial
distance of the regions.
In Fig. 2, we show an example of a spectrum, while the complete results of the spectral analysis are reported in Fig. 3 for
the 5 spectral regions, displayed in order of increasing distance
from the putative center. The temperature does not show large
variations, indicating that thermal conduction must be efficient.
The innermost regions are characterized by a higher metal abundances for Ne, Mg and Si. In these cases, the profiles show a
decreasing trend with increasing distance from the center. While
Ne and Mg retain super-solar abundances in most regions, Si is
above solar abundance only in the two innermost regions. On
the other hand, S and Fe appear to have more uniform abundances, about 1.0 and 0.3 respectively. The thermodynamical
parameters of both the components agree in general with the
findings of Troja et al. (2006). The abundances patterns are generally consistent with the ones found by Troja et al. (2008) in
the south region of the remnant, but smaller in value than those
of the remarkable ejecta ring surrounding the pulsar wind nebula found by them. Troja et al. (2008) also compared the Mg/Si,
S/Si, and Fe/Si ratios of the ring with the predictions of both the
Fig. 3. Results of the spectral fittings to the hard thermal component
obtained in the 5 iso-brightness regions in IC 443. See Fig. 1 for the
position of the contours levels used in the region definition. Abundances
are relative to solar values. The abscissa contains the average distance
from the center (horizontal error bars are standard deviations). Vertical
error bars are 2σ uncertainties. The normalization of the spectrum is
expressed in terms of n2 l, where n is the plasma density and l is its
extension along the line of sight.
nucleosynthesis model of core-collapse (Woosley & Weaver
1995) and type Ia (Badenes et al. 2003) supernovae, finding
closer agreement with the former model. We verified that Mg/Si,
S/Si and Fe/Si ratios found by us in the innermost spectral region
in Fig. 1 agree with those in the ring region, and thus we concluded that our results also support the idea that IC 443 had a
type II progenitor.
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3. G166.0+4.3 (VRO 42.05.01)
G166.0+4.3 is a supernova remnant in the anti center direction,
whose X-ray and radio morphology inspired considerable interest. Burrows & Guo (1994) and Guo & Burrows (1997) analyzed the ROSAT and ASCA X-ray observations of this remnant,
finding a centrally peaked morphology that appeared to be completely enclosed within the radio emission of this object. The
radio emission, in turn, has a limb-brightened double-shell morphology, which has a small diameter half-shell at NE (about 30 ,
a.k.a. the shell) and a larger incomplete shell at SW (a.k.a. the
wing, Pineault et al. 1985; Pineault et al. 1987; Leahy & Tian
2005, and references therein).
G166.0+4.3 was observed by XMM-Newton on
21 February 2003 (ID 0145500101) and on 23 February
2003 (ID 0145500201). The first observation was pointed at the
western part of the remnant (the “wing”), and had a total PN
exposure time, after the screening procedure, of 5.0 ks, while
the second targeted the small incomplete shell at the east. This
latter pointing was severely contaminated by high energy proton
flares and the net PN exposure time is only 1 ks, and was not
used in the spectral analysis.
A mosaic map of this remnant in the band 0.3–5.0 keV,
obtained using the two XMM-Newton archive observations, is
shown in Fig. 4, which also includes the 1420 MHz radio map
of the Canadian Galactic Plane Survey (Taylor et al. 2003). The
X-ray morphology derived by previous observations was confirmed, and the bright region (named “western bright knot” by
Burrows & Guo 1994) was resolved for the first time, showing
an elongation in the NW-SE direction. In the light of its radio
and X-ray morphology, this remnant may be considered part of
the sub-class of mixed-morphology (MM) SNRs introduced by
Rho & Petre (1998). The explanation of the peculiar radio morphology dates to Pineault et al. (1987) and involves an explosion in a moderately dense medium, followed by the breaking of
the shock in a rarefied hot tunnel, and by the interaction with a
denser medium again. Guo & Burrows (1997) made the so far
only measurement of metal abundances in the X-ray spectrum
of G166.0+4.3, finding a substantial underabundance of Mg, Si,
and Fe in the western bright knot and in the SE and NE of the
wing.
To study the abundances in the centrally peaked region of
G166.0+4.3, we tried to characterize the thermal properties in
the plasma of this remnant by selecting 4 interesting regions for
spectral analysis, which are shown in Fig. 4. Regions 1, 2, and
3 are similar to regions 1, 2 and 3 of Guo & Burrows (1997),
but are considerably smaller and point towards bright features,
and are therefore much less affected by background (also considering the smaller XMM-Newton PSF compared to the ASCA
one). In each region, we fitted the X-ray spectra with a Mewe
et al. (1985) spectral model modified by interstellar absorption,
allowing the abundances of O, Ne, Mg, Si, S, and Fe to vary.
In Fig. 5, we show, as an example, the spectrum of region 1.
The results of the spectral analysis, presented in Fig. 6, indicate
small variation in temperature (of up to 10%) and large variation
in interstellar absorption (of up to 50%). The variation patterns
appear to agree more closely with those found by Burrows &
Guo (1994) than those of Guo & Burrows (1997). The regions
appear to be characterized by an underabundance of O and Si
and an overabundance of S, while Ne, Mg, and Fe appear to
have about solar abundance values. Our Mg, Si, and Fe abundance measurements agree with those of Guo & Burrows (1997),
who also measured subsolar abundances of these elements. A
NEI thermal model does not improve the fits for these regions,
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Fig. 4. Top: XMM-Newton EPIC images of G166.0+4.3 in the 0.3–
5 keV background subtracted and vignetting corrected. Black thick contour is at 0.2 × 10−3 cnt s−1 pix−1 and black thin contours correspond to
0.6, 0.8, 1.0, 1.2 and 1.5 × 10−3 cnt s−1 pix−1 (1 pixel = 12 × 12 ).
Numbers mark the spectral extraction region used in the text (Region 1
is defined by the outermost thin surface brightness contour). The XMMNewton EPIC field of view is marked in white (two observations combined). Bottom: 1420 MHz CGPS DRAO radio image of G166.0+4.3
(adapted from Leahy & Tian 2005). X-ray contours of the top panel are
shown in black.
> 1012 s cm−3 and abundances similar
giving a ionization time τ ∼
to those obtained with the equilibrium model.
We investigated whether a radial gradient in metal abundances exists inside the bright region 1 of the remnant (the
“western bright knot”), in a similar way of the analysis in Sect. 2
for IC 443, but, due to the low quality of the data in this case, the
results do not provide tight constraints on metal abundances.
F. Bocchino et al.: Metal abundances in MM SNRs
143
Residuals
Counts s−1 keV−1
−2 0
2 0.01
0.1
1
G166.0+4.3 Region 1
PN (black), MOS1 (red) and MOS2 (green)
1
Energy (keV)
2
Fig. 5. XMM-Newton EPIC spectrum of the G166.0+4.3 Region 1 in
Fig. 4. The best fit thermal model is shown as continuous line (with
residual in lower panel).
4. Discussion
In Lazendic & Slane (2006) IC 443 and G166.0+4.3 were listed
as standard abundances MMSNRs. However, the spatially resolved spectral analysis we did on the bright X-ray central regions of IC 443 confirmed that this object belongs to the subclass of metal-rich mixed-morphology remnants. In the case of
G166.0+4.3, no compelling evidence of enhanced metal abundances exists apart from sulfur in the outer regions. The short exposure time of the XMM-Newton observation of the G166.0+4.3
prevented us from studying possible trends in the metal abundances of the remnant core. Our results suggest that we should
review the other MMSNRs in the list of Lazendic & Slane (2006)
to better measure chemical abundances of all known remnant of
similar type. Other MMSNRs may be erroneously cataloged as
solar-type abundances objects.
It is not easy to determine whether the high metal abundances measured in some MMSNRs are typical of this class, or
if they correspond to an evolutionary phase in the life of these
objects, or depend on the initial remnant composition. From the
theoretical point of view, both the evaporating-cloud model of
White & Long (1991) and the radiative SNR model Cox et al.
(1999), used traditionally to explain the centrally peaked thermal X-ray morphologies, are of limited help in understanding
the subclass of metal-rich MMSNRs, since they do not consider
in detail the mixing between ejecta and shocked material inside
the remnants. However, these models have often been used to
interpret the X-ray emission of some MMSNRs (e.g., Bocchino
& Bandiera 2003; Lazendic & Slane 2006), and should therefore
be compared with our own results in understanding their limitations.
We first consider the evaporating-cloud model of White &
Long (1991), in which the X-ray central enhancement is due
to material ablated by shock-heated clouds inside the remnant.
The models depend on the traditional parameters E (the explosion energy), ρ (the preshock ISM density), and t (the age
of the remnant), and two parameters describing the evaporating clouds, namely C (the ratio of mass in clouds to mass in
Fig. 6. Spectral fitting results of regions 1–4 in G166.0+4.3. See Fig. 4
for the position of the regions. Black arrows indicates a lower limit of 0.
the intercloud medium) and τ (the ratio of the cloud evaporation time-scale to the SNR age). With an appropriate choice of
normalized quantities for the radial profiles (for our study, we
consider the density normalized to the post-shock density, ρ/ρ s ,
the temperature normalized to the shell temperature T/T s, and
the normalized emission measure EM(πR2s )/EM(tot); see their
Fig. 4), they show that the dependence on E, ρ0 , and t can be
masked out in the presentation of the results. To ease the comparison with observations, the authors discussed the asymptotic
behavior of their model when C and τ → ∞, in which case the
model only depends on the ratio C/τ. In Fig. 7, we show the
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Fig. 7. Comparison between the White & Long (1991) evaporatingcloud model and the observed profiles in IC 443 and G166.0+4.3. Top:
normalized emission measure (see text for explanation). Middle: density normalized to central value. Bottom: post-shock temperature normalized to central value. Black and red crosses are IC 443 low and
high temperature component data with uncertainties, respectively; blue
curves are G166.0+4.3 data. Black curves are White & Long (1991)
profiles for models with C/τ = 0 (dashed), 1, 2, 3 and 4. The distance
from the center is normalized to the shell radius (assumed 10 for IC 443
and ∼6.5 for G166.0+4.3, see text for details).
normalized temperature, density and emission measure for the
models with C/τ = 0, 1, 2, 3 and 4. To ease the comparison with
the data further, we decided to normalize the temperature and
density to their respective central values, instead of their shell
values, as in the work of White & Long (1991). In our observations (and in general in all MMSNRs observations), the low
X-ray surface brightness of the shell prevents an accurate spectral analysis from being possible, so it is particularly difficult
to estimate the shell values. On the other hand, the central regions are by definition the brightest, and accurate results may be
achieved here. An additional concern is the shell radius used to
normalize both the abscissa of the profiles and the emission measure. Both IC 443 and G166.0+4.3 have morphologies that are
far from being circular symmetric and offset X-ray peaks, so for
each of these objects, it is particularly difficult to estimate a single shell radius. For IC 443, we adopted a radius of 10 (∼4.3 pc
at 1.5 kpc distance), which is the average distance of the radio
shell from the X-ray peak in the East, North, and West of the
remnant. For G166.0+4.3, we used ∼6.5 (∼13 pc at 4.5 kpc distance), which is the distance to the east radio wing.
In Fig. 7, we also plot the derived values for the low and
high temperature component of IC 443 and the single thermal
component of G166.0+4.3. The profiles of G166.0+4.3 appear
to be steeper than the profiles of IC 443. In general, there is a
good agreement between the normalized emission-measure and
that of the model with C/τ = 2−3 for both objects, even if the
uncertainties in the adopted shell radius can affect the final results. However, we emphasize that the density and temperature
profiles are not entirely consistent with the emission measure
profiles. In case of IC 443, the density profile would be in closer
agreement with C/τ ≥ 4, while the temperature profile would
be with C/τ < 1. An inconsistency is present in both the thermal components of IC 443, which also holds for the density of
G166.0+4.3 implying that C/τ > 4. Conversely, we could use
the density profile to find a good fit to the model by varying the
shell radius within a ±50% range, but the emission measure and
temperature profile would remain inconsistent. We verified for
instance that the emission measure would be far more centrally
peaked than observed. Finally, we note that the central peak in
both the observed density profiles seems to exclude evaporatingcloud models with low values of τ (cf. Fig. 1 in White & Long
1991).
In summary, the comparison with the evaporating-cloud
model infers high values of τ, but does not constrain tightly the
true actual value of C and τ; it also indicates that the observed
density profiles are usually steeper than predicted by the model
reproducing the emission measure profiles more successfully, a
situation that cannot be explained in terms of an ejecta component on top to the evaporating-cloud component. If it was so, in
fact, we would expect the density profile to be shallower than
the model, because of the enhancement in density provided by
the ejecta. We therefore conclude that the X-ray morphology of
these remnants is inconsistent with the evaporating-cloud model.
The temperature profiles, although not entirely consistent with
the model, appear to indicate very limited variations, and therefore points toward efficient thermal conduction inside both of
the remnants. The failure of the cloud evaporation model and
the high efficiency of thermal conduction suggest an explanation in terms of the entropy-mixed thermally conductive model
developed by Shelton et al. (2004) to explain the observational
data of MMSNR W44, possibly amended to take into account
the peculiar environment in which these remnants expand. A detailed numerical model of the metal-rich MMSNRs that takes
into account mixing between the ejecta and shocked ISM material, thermal conduction, and a realistic model of the environment, is required to verify this scenario.
The comparison with the radiative model of Cox et al. (1999)
can be completed more straightforwardly following Lazendic &
Slane (2006) by assuming that the current shock radius is the radius at which the remnant enters the radiative stage. Following
the relations of Cox et al. (1999), this yields a lower limit to
the ambient and central densities. Using realistic radius values, as used in our comparison with the cloudy ISM model, we
derive ambient density lower limits of the order of 10 cm−3 .
While it is true that, in both remnants, there is substantial evidence of expansion in dense environments, it is also unlikely
that the expansion always occurred inside a medium so dense.
For IC 443, Troja et al. (2006) argued that the remnant evolved
inside wind-blown bubble, hypothesis originally suggested by
Braun & Strom (1986), while for G166.0+4.3, Gaensler (1998)
argued that the remnant had expanded into a low-density hot
F. Bocchino et al.: Metal abundances in MM SNRs
tunnel before encountering recently a high density region. The
hot tunnel location was inferred to coincide with that of the Xray emission peak. In both cases, it is unlikely that a radiative
model assuming a long evolution inside a dense medium is applicable. This is also the case whenever there is evidence of a massive progenitor star, which usually have strong pre-supernova
winds. Chen et al. (2008) proposed that the X-ray emission of
Kes 27 MMSNR could be explained in terms of a shock reflected
by a cavity wall, a mechanism that may operate also in IC 443
and G166.0+4.3, given the observational evidences about their
environments.
5. Summary and conclusion
We analyzed on XMM-Newton X-ray observations of the two
supernova remnants IC 443 and G166.0+4.3, focussing on the
nature of their centrally peaked thermal X-ray emission above
1 keV. We confirmed that the X-ray morphology of these remnants implies that they belong to the class of mixed morphology
SNRs. In contrast to previous results, we measured the chemical abundances of the bright central peak of IC 443 to be above
solar, which infers that this object is a member of the subclass
of metal-rich MMSNRs. For G166.0+4.3, there is no evidence
of enhanced metal abundances, except maybe for sulfur in the
outer regions, but the limited quality of the data prevented us
from performing spatially resolved spectroscopy of the central
peak.
We derived profiles of metallicity, temperature, density, and
emission measure, and compared them with the predictions of
the White & Long (1991) and Cox et al. (1999) models, traditionally used to explain the MMSNR morphology in other objects. We found that the observed profiles are inconsistent with
the models, and we argued that more detailed modeling including the mixing of ISM and ejecta material is required to explain
the observations more accurately. The entropy-mixing model of
Shelton et al. (2004) appears to be the most promising starting
point for more detailed modeling, although the effects of the
complex environment in which the MMSNRs seem to be located
(Lazendic & Slane 2006) and the projection effects discussed by
Petruk (2001) should be taken into account. Temperatures, density, and chemical abundances profiles of MMSNRs are therefore the key observational constraints to be compared with future
models.
Acknowledgements. We thank D. Leahy for providing us with the 1420 MHz
total intensity map of IC 443 in electronic format. We also thank O. Petruk and
145
S. Orlando for useful comments on this work. This work makes use of results
produced by the PI2S2 Project managed by the Consorzio COMETA, a project
co-funded by the Italian Ministry of University and Research (MIUR) within
the Piano Operativo Nazionale “Ricerca Scientifica, Sviluppo Tecnologico, Alta
Formazione” (PON 2000-2006). More information is available at http://www.
consorzio-cometa.it.
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